APP-V can be transformed to APP-I and APP-II, and APP-II can also be transformed to APP-V. However, the crystalline transformation process is not well known. By process of diammonium hydrogen phosphate, the intermediate APP-X in between ammonium polyphosphate form II (APP-II) and form V (APP-V) was found and measured. The transformation process from APP-X to APP-V were studied and discussed. The XRD patterns in this process were discussed in detail so that the crystalline transformation process from APP-X and APP-V can be understood.